The present invention relates to a dot matrix printing head, and more particularly, to a control circuit for a dot matrix permanent magnet printing head.
It is known that serial printers, using printing needles or the like, are widely spread on the market. In such printers several electromagnets are selectively energized, each one for causing the impact of a printing element, usually a needle, against a printing support. Two kinds of printing heads are basically used, the one with a simple electromagnet and the one with a permanent magnet.
In the simple electromagnet type print heads, each electromagnet is normally deenergized. The energization causes the attraction of an armature which, in turn, causes the movement of the printing element. In the permanent magnet type print heads, a permanent magnet maintains a plurality of resilient armatures in attracted and bent position. A winding, coupled with the permanent magnet circuit, is associated with each one of the armatures. The selective energization of the various windings causes the neutralization of the magnetic field produced by the magnet on the related armatures, and the release of the related armatures, which in turn causes the movement of the related printing elements.
The performances attainable by these print heads are heavily dependent on the control circuits which cause their energization and on their mechanical characteristics. In order to obtain high performances it is required to impart to the energization (or demagnetization) windings a high current in a very short time, to maintain such current for a suitable time, and then to remove such current in a very short time. The energization cycle defines, but for a certain hysteresis, the mechanical displacement cycle of the armature, at the end of which the armature returns to its rest position. However, when the armature reaches its rest position, it is affected by a remarkable speed imparted by the returning means (resilient or magnetic). Therefore it tends to strike against a stop element and to rebound with an oscillatory phenomena which, in spite of the damping elements usually provided, end in a settling period which normally has a duration not lesser than the duration of the energization cycle. The armature vibration and its impact against the stop element are a further cause of noise, whose intensity is greater the greater the kinetic energy, that is, the armature speed and consequently the vibration amplitude.
The requirement to have a repetitive and uniform behavior in the course of subsequent printing operations imposes that an armature must be energized when it is in stable rest position, thus an actuation period not less than the sum of the energization cycle duration and the settling period duration. The performances of the dot printing heads are therefore limited by energization cycle duration and by the settling time. Several arrangements have been proposed. On one side they aim to shorten the energization cycle duration by means of energization circuits which produce current pulses of rectangular shape. On the other side they aim to shorten the duration of the settling period by means of mechanical dampeners, pneumatic, resilient dampeners or the like.
It is the object of the present invention to provide a circuit which not only allows for the generation of very short energization cycles, with current pulses very close to a rectangular shape, but also allows for a reduction of the settling period of the armatures in a printing head, as well as a reduction of the noise they produce, by performing a damping action which may cumulate with the one provided by other possible devices.
A further object of the present invention is to provide a driving circuit which may control a dot matrix printing head to obtain quality print characters designed according to a high resolution matrix, the circuit being simple and inexpensive.
These results are achieved by providing a driving circuit where a plurality of windings may be selectively energized by individual control circuits, while a common transistor switch, periodically closed, periodically establishes and interrupts a current recycle path which maintains the energization current for a preestablished time interval, interrupts it when required, then enables, with a next reclosure, the circulation of induced currents which have a damping effect on the armatures. The closing/opening of the common transistor switch, does not affect the energization of the windings even if the energization period is longer than the closing/opening period of the common transistor. This allows for the execution of impressions according to a dot matrix having an high resolution.
These and other features of the invention and its advantages of the present invention will become more apparent from the following description of a preferred form of embodiment of the invention and of some variants thereof.